Protective system for capacitance serially connected with inductive apparatus



May 31, 1966 R. J. RADUS ETAL PROTECTIVE SYSTEM FOR CAPAC ITANCESERIALLY CONNECTED WITH INDUCTIVE APPARATUS Filed Aug. 12, 1965 Fig.3.

4 $heets-Sheet 1 LOAD LOAD LOAD LO AD LOAD INVENTORS Raymond J. Rodusand John J. Asfleford,Jr.

dmbui ATTO R NEY May 31, 1966 R. J. RADUS ETAL PROTECTIVE SYSTEM FiledAug. 12 1963 FOR CAPACITANCE SERIALLY CONNECTED WITH INDUC'I'IVEAPPARATUS 4 Sheets-Sheet 2 V max Vc Fig.4A.

B I M M V II I --V mc|x [A A A H948 0 p May 31, 1966 R. J. RADUS ETAL3,254,268

PROTECTIVE SYSTEM FOR CAPACITANCE SERIALLY CONNECTED WITH INDUGTIVEAPPARATUS Filed Aug. 12 1963 4 Sheets-Sheet 5 Fig.9.

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PROTECTIVE SYSTEM FOR CAPAC ITANCE SERIALLY CONNECTED WITH INDUCTIVEAPPARATUS Filed Aug. 12 1965 4 Sheets-Sheet 4.

United States Patent f PROTECTIVE SYSTEM FOR CAPACITANCE SERI- ALLYCONNECTED WITH INDUCTIVE APPARA- TUS Raymond J. Radus, Monroeville, andJohn J. Astlcford, Ji'., Sharon, Pa., assignors to WestinghouseElectricCorporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Aug.12, 1963, Ser. No. 301,500 11 Claims. (Cl. 317-14) This inventionrelates in general to electrical apparatus, and more particularly toprotective systems for electrical apparatus.

Voltage regulation of electrical distribution systems has always been aparticularly difficult problem. One method employed to improve thevoltage regulation in an electrical distribution system is to providetransformers with tapchanging apparatus, and either automatic or manualmeans for operating the tap-changing equipment. This method is expensivecompared to the cost of the associated transformer and has thedisadvantage of allowing voltage to only be changed in steps, plus theadditional maintenance produced by the tap-changing system.

Another method of voltage regulation involves connecting a capacitor inseries circuit relation with the primary winding or the windingconnected to the alternating potential source, of the transformer. Thevoltage drop across the capacitor varies with the load current beingcarried by the transformer and compensates for at least. a portion ofthe voltage drop across the overall impedance of said transformer, andfor at least a portion of the voltage drop across the feeder lineconnected to said transformer in an electrical distribution system. Thismethod, however, was not practical for several reasons. One of thereasons is the abnormally large and distorted exciting currents that maybe produced when a saturable inductance, such as a transformer, isconnected in series circuit relation with a capacitor. The largecurrents produced by this phenomenon, which is commonly calledferro-resonance, are not transients, but persist as a steady statecondition until the circuit is interrupted or the equipment is damaged.Another reason involves sub-synchronous motor operation. Under certainconditions an induction motor supplied through a power line containing aseries capacitor will operate as a generator, supplying current of lowerthan line frequency, with the excitation being supplied by thecapacitor. Thus, large sub-synchronous currents are generated due to thereduced reactance of the supply circuit to the lower frequency,resulting in large voltage swings. This phenomenon will generally occurat reduced motor speeds, such as during start-up or during overloads,causing the motor to lock into step at subsynchronous speed, vibrateexcessively, produce large current pulsations and voltage swings.

Further, the high voltages that may be built up across the capacitor dueto load changes and short circuit current made the'capacitor too costly,and an inexpensive means for limiting the capacitor voltage to a valuewhich would make the capacitor cost attractive was not avail-, able.

With the advent of the type of transformer which utilizes metallic stripor foil to form its windings and which has at least a portion of itswindings interleaved or arranged to form a predetermined capacitancebetween said windings, which is effectively connected in series circuitrelation with the primary winding and therefore aiding voltageregulation, the problem of an inexpensive means to effectively dampenferro-resonance and subsynchronous motor operation, and to limit thevoltage across the capacitive portion of the transformer has be-3,254,268 Patented May 31, 1966 1 V-afzdi where i is the primary currentof the transformer thus, load changes, overloads and short circuits willproduce high capacitive voltages.

Another factor which must be considered when utilizing a seriescapacitor is that unless some provision is made to remove thecapacitance from the circuit upon short circuit, tremendous shortcircuit currents will flow. This is due to the fact the inductivereactance of the transformer has been reduced or cancelled by thecapacitive reactance.

Protective circuits of the prior art for series connected capacitorshave disadvantages in not protecting the capacitance and associatedapparatus against all of the abnormal conditions. that may be producedby series connected capacitance. For example, the protective system mayadequately protect the capacitance against over voltage due to loadchanges, but may not dampen ferroresonance, allowing associatedtransformer apparatus to be damaged. Or, if the protective systemdampens ferroresonance, the system may be deficient in not damping thecircuit conditions which cause sub-synchronous operation of motorsconnected to the'power distribution system.

Therefore, it is desirable to have an inexpensive, static protectivemeans for quickly damping unstable circuit conditions, such asferro-resonance and sub-synchronous motor operation, and for shortingout the effect of the capacitance when the voltage across saidcapacitance reaches a predetermined magnitude. Further, the protectivemeans must be able to continuously provide protection withoutdeterioration due to repeated operation. of the protective means, andthe protective means must allow the transformer to be restored to normaloperation upon the termination of an abnormal condition.

Accordingly, it is an object of this invention to provide new andimproved protective means for electrical apparatus.

Another object of the invention is to provide a new and improvedprotective system for inductive apparatus.

Another object of this invention is to provide a new and improvedprotective system for capacitive apparatus.

A further object of this invention is to provide new and improvedprotective means for a system utilizing inductive apparatus, and seriesconnected capacitance to aid voltage regulation.

Still another object of this invention is to provide a new and improvedprotective system for inductive apparatus of the type which has itswindings arranged to provide a series capacitance which effectively aidsvoltage regulation.

Another object of this invention is to provide a new and improvedprotective system for power distribution systems utilizing seriesconnected capacitance which prevents over voltages due to load changesand short circuits and which dampens unstable circuit conditions.

Briefly, the present invention accomplishes the above cited objects byproviding a protective system for power distribution systems whichutilize series capacitance, and for inductive apparatusof the type whichutilize series capacitance in the primary circuit, such as transformers.The capacitance in the power distribution system may be used to regulatethe feeder voltage to a plurality of distribution transformers. Thecapacitance in the primary circuit of an individual transformer may beeffective capacitance, produced by certain types of windings and windingarrangements, or actual capacitance produced by physically connecting acapacitor in the primary circuit of said transformer. In general, theprotective system shunts the capacitor, or capacitor section of thetransformer windings, at the instant the voltage across the capacitorexceeds a predetermined maximum. The capacitance is discharged by theshunting action of the pro tective system, and then the shunt-ingact-ion automatically ceases, allowing the capacitor to recharge.

If the condition which caused the high current and consequently the highvoltage across the capacitance still exists, the capacitance willrecharge and the protective system will again shunt the capacitance atthe predetermined maximum voltage, causing the capacitance to againdischarge. This action continues until the abnormal condition ceases,allowing normal operation with the capacitance connected in the circuit,or until other protective apparatus such as circuit breakers or fusesassociated with the transformer removes the transformer from the feedercircuit.

More specifically, the protective system comprises a pair of electrodes,with a predetermined spacing or gap between them, connected across or inshunt with the series capacitance. When the voltage across thecapacitance, and hence across the electrodes, reaches a voltage whichcauses an are between the pair of electrodes, the capacitance will beeffectively shunted from the inductive circuit, causing the capacitanceto disharge, and allowing the full leakage reactance of the inductiveapparatus to oppose the flow of current. In order to prevent the arcfrom continuing after the capacitor has substantially discharged, thusdamping any unstable circuit conditions, as

well as preventing burning and pitting of the electrodes, which wouldcause unpredictable operation on subsequent protective operations, amagnetic field is disposed perpendicular to the arc. The magnetic fieldcauses the arc to move across the face of the electrode and henceprevents the arc from overheating any one particular area or spot. Thesize of the electrodes are such that the heat produced by the arc andcurrent flowing through the electrode is insufficient to substantiallyaffect the electrode spacing or gap, and insufficient to initiatethermionic emission which reduces the breakdown voltage of the gap.Thus, the maximum voltage at which the protective system will operateu-pon repeated operations is substantially unaffected. Morespecifically, the size of the electrodes depends upon the speed of thearc across the face of the electrodes. The faster the movement of thearc, the longer the electrodes will have to be, but the depth of theelectrodes can be lessened, as the heating of any one particular areahas been reduced. The proper depth and length of the electrodes are veryimportant in that they must not be too small. To obtain the unusual,unexpected results of the protective system, the depth must provide aheat sink which is adequate in preventing excessive heating, and thelength must be sufficient to :blow out or extinguish the arc.

The are power required to sustain the arc must be maintained at amaximum, thus starving the arc and quenching it immediately upondischarge of the capacitance, to prevent follow current from theelectrical system from flowing through and sustaining the arc. The rapidquenching of the arc is due to the combination of maintaining theelectrodes at a relatively cool temperature preventing thermionicemission, and the rapid movement of the are over a sufiicient length ofelectrode. An are moving rapidly over a relatively cool surface requiresa maximum of energy to sustain the arc. Therefore, when the capacitancedischarges, the arc extinguishes, as the energy required to sustain thearc is greater than available from the circuit. I

Further objects and advantages of the invention will become apparent asthe following description proceeds and features of novelty whichcharacterize the invention an embodiment of the invention;

FIGS. 2 and 2A are schematic diagrams illustrating another embodiment ofthe invention;

FIG. 3 is a schematic diagram illustrating another embodiment of theinvention; v

FIGS. 4A, 4B, and 4C illustrate graphically certain voltage-currentrelationships explanatory of the operation of the invention;

FIG. 5 shows oscillograms illustrating abnormal capacitive voltage andcapacitive discharge current when a transformer is excited throughseries capacitance;

FIG. 6 is a schematic diagram illustrating another embodiment of theinvention;

FIG. 7 is a side elevation, partially in cross section, illustrating oneembodiment of the invention;

FIG. 8 is a front elevation, partially cut away, of the embodiment ofthe invention shown in FIG. 7;

FIG. 9 is a top view, with the cover removed, of the embodiment of theinvention shown in FIGS. 7 and 8;

FIG. 10 shows a front elevation of a transformer, partially cut away andpartially schematic, showing one method of mounting the protectivesystem of FIGS. 7, 8 and 9 relative to a transformer;

FIG. 11 is a front elevation, in section of another embodiment of theinvention; and

FIG. 12 is a front elevation, in section of still another embodiment ofthe invention. 1

Referring now to the drawings, and FIG. 1 in particular, there is showna schematic diagram illustrating the protective system 10 connected toprevent the voltage across the capacitive section 12 of transformer 14from exceeding a predetermined maximum. The construction oftransformers, such as shown schematically in FIG. 1, whereby apredetermined capacitance is formed by various types of windings andwinding arrangements, such as interleaving sections of foil constructedprimary windings, is described in a copending application by H. W. Book,Serial No. 248,839, filed December 27, 1962, and assigned to the sameassignee as the present application.

FIGURE 1 illustrates a transformer having primary winding sections 16and 18 and secondary winding 20 inductively disposed on a magnetic core22. The primary sections 16 and 18 have certain portions interleaved toform a predetermined capacitance 12 between the windings and insulation.The capacitance 12 is effectively connected in series circuit relationwith primary winding 18 from terminal 24 of primary winding 16 toterminal 26 of primary winding 18, and therefore, the primary currentflowing from alternating current input terminals 28 and 30 flows throughcapacitor 12. The primary current flowing through series capacitance 12produces a voltage across capacitance 12 which offsets or cancels atleast a portion of the voltage drop across the effective inductance ofthe transformer, aiding voltage regulation.

Some protective means, however, must be provided to dampen unstablecircuit conditions, such as ferro-resonance and sub-synchronous motoroperation, and to limit the maximum voltage across capacitance 12 andthe portion of the primary windings 16 and 18 arranged to produce saidcapacitance. The capacitive voltage can reach dangerous and damagingmagnitudes when the primary current increases, such as during inrushtransients, load changes or steps, and short circuits. It is essentialthat the capacitance or capacitor section of the transformer beprotected from the overvoltages to prevent puncture or failure ofinsulation.

The protective means should be inexpensive to manufacture, since thedistribution transformers they will be called upon to protect arerelatively inexpensive, and the protective means must restore the effectof the capacitance to the circuit when unstable circuit conditions haveceased and the capacitor voltage drops below a predeter mined minimum.Further, the protective action should be repeatable withoutsubstantially impairing the effectiveness of the protective means, andshould provide protection instantly during the first half cycle ofover-voltage.

A protective means having the desired characteristics is shownschematically in FIG. 1 at 10. Protective means is connected toeffectively shunt the effect of the capacitance 12 and protectcapacitance 12 from overvoltage -when the voltage drop across saidcapacitance reaches a certain magnitude. Since, as hereinbefore stated,the equivalent circuit .of FIG. 1 would show capacitance 12 lumped intoone capacitance and connected between terminals 24 and 26 as shown inFIG. 2, the protective means 10 may be connected to terminals 24 and 26.

In general, protective means 10, which is shown in an enlarged view inFIG. 1A, comprises electrodes 32 and 34 connected to terminals 24 and26, through conductors 36 and 38, respectively. Electrodes 32 and 34,which may be blocks of copper, or any other suitable conductivematerial, are disposed with similar faces adjacent to each other to forma small air gap or spacing 40. The size of gap 40 is determined by thevoltage across capacitance 12 at which it is desired to shunt saidcapacitance. The breakdown voltage across gap 40 is a direct function ofthe gap length at substantially 140 volts per .001" in the range of.010" to .015". For example, at a gap length of .015"; the gap breakdownvoltage is substantially 2000 volts peak. To prevent a substantialchange in breakdown voltage due to the heating and expansion ofelectrodes 32 and 34 after several successive operations or voltagebreakdowns, the electrode should have a depth *sufiicient to provide asubstantial heat sink effect. For example, in one applicationrectangular-blocks of copper 3 long, /2 wide and 1%" deep have beenfound to provide an adequate heat sink to prevent the air gap fromsubstantially changing due to the expansion of the electrode, as well aspreventing thermionic emission and burning and pitting of theelectrodes.

The surfaces of the electrodes 32 and 34 which face each other to formthe gap 40 may be substantially flat, or they may be slightly taperedfrom the center of the facing surfaces to the edges, or may have anyother suitable configuration.

The use of electrodes 32 and 34 alone, however, to shunt capacitance 12upon the voltage across the capacitance 12 reaching a certain maximummagnitude is not sufficient. The are will not extinguish when thecapacitance 12 discharges, but continues to carry the current flowingthrough primary winding 18 until the current alternation goes to zero.If the arc is not quickly extinguished after the capacitance discharges,unstable circuit conditions such as ferro-resonance and sub-synchronousmotor operation are not dampened. Further, the heat produced by the arcis sufficient to melt the electrodes with resulting pits and burrs ifleft to concentrate on one small area of the electrode face. Thus,accurate repeatability of the operation of the protective system 10would not be possible, and the electrodes would have to be replacedafter only a few voltage discharges across the gap, and the transformerand associated apparatus may be damaged since ferro-resonance andsub-synchronous motor operation would continue. In order to prevent theare from continuing after discharge of capacitance 12, and to preventconcentration of the are on a small surface area of the faces of theelectrodes 32 and 34, a magnetic field is created, which issubstantially perpendicular to the are, by magnet members 42 and 44.Magnet members 42 and 44, which are disposed on opposite sides of gap 40with their respective poles being arranged to attract each other, may bepermanent magnets or electromagnets, with permanent magnets beingpreferred since they do not require any external connection.

By introducinga strong magnetic field across gap 40 by magnets 42 and44, when an arc is produced across electrodes 32 and 34, the arc willmove across the surface of the electrode due to the reaction between themagnetic field and electric current. The arc is like a conductor ofelectricity in an electric motor which moves when a current is passedthrough it while it is being subjected to a mag netic field. Theintensity of the magnetic field and are current will determine how fastthe arc moves. The intensity of the magnetic field, however, is notcritical, requiring substantial changes in field intensity to producenoticeable changes in arc movement. For example, in one applicationutilizing a 2000 volt gap, a magnetic field of 1000 gauss was found togive sufiicient arc mobility. However, situations where a stronger fieldis required will be examined hereinafter. With the movement of the arcacross the adjacent surfaces of electrodes 32 and 34, no overheating isexperienced and the energy required to sustain thearc is maintained at amaximum. The are is promptly extinguished after the capacitance 12 hassubstantially discharged as the circuit is unable to supply the energyrequired to sustain the arc. The electrodes are, therefore, able toperform over long periods with accurate repeatability and little or nodamage to the adjacent surfaces forming the gap 40.

In describing the operation of protective means 10 and how it protectstransformer 14 by shunting the capacitance 12 when the capacitivevoltage reaches a predetermined maximum, references will be made to thegraphical representations shown in FIGS. 4A, 4B, and 4C. FIG. 4A is agraph illustrating the voltage waveform across the capacitance 12, and,therefore, across the gap 40 of electrodes 32 and 34, for varioustransformer primary current magnitudes, as shown in FIG. 4B. Asillustrated, the first cycle of primary current I produces a volt-age Vacross capacitance 12 which is within the maximum capacitor voltage Vmax., as determined by the spacing of electrodes 32 and 34 of protectivesystem 10. The second and third cycles of primary current I increase inmagnitude sharply, which tends to produce a capacitive voltage drop Vwhich would exceed the maximum capacitive voltage V. max., as indicatedby the dotted portions of the second and third cycles of voltage -V inFIG. 4A above line V max. The voltage V rises along the sine wave untilthe maximum capacitive voltage V max. is reached, at which point thevoltage across gap 40 breaks down and establishes an are betweenelectrodes 32 and 34. The capacitance 12 quickly discharges, droppingthe capacitive voltage V to substantially zero at point 52. The magneticagitation or movement of the are due to magnet members 42 and 44cooperates with the heat sink effect of the electrodes 32 and 34 toextinguish the arc, and the capacitance 12 immediately starts torecharge until the capacitive voltage V again reaches V max. at point54. The capacitance 12 discharges to point 56, the arc is extinguishedby the magnetic movement of the arc, and the capacitance again chargesto the maximum capacitive voltage V. at point 56. The capacitance 12again discharges to point 58 and begins to charge again, but this timethe voltage sine wave envelope has dropped below the maximum capacitivevoltage V allowing the capacitive voltage to only reach point 60. Atpoint 60 the capacitance discharges and follows a sine waveconfiguration to zero and polarity reversal, where the same processcontinues in the negative portion of the cycle. The number of times themaximum voltage V max. is reached during a half cycle is a directfunction of current magnitude, as evidenced by formula This protectiveaction of protective system 10 continues as long as the capacitivevoltage tends to exceed the maximum capacitive voltage V as determinedby the protective system.

FIG. 4C illustrates the voltage V across capacitance 12 utilizing aprotective system similar to protective system 10 except with the magnetmembers 42 and 44 relow the maximum capacitive voltage V max.

moved. The voltage during the second cycle rises until the maximumcapacitive voltage V max. is reached at point 62, which causescapacitance 12 to discharge and drop the voltage across capacitance 12to substantially v zero at point 64. However, instead of the arcextinguishing, as shown in FIG. 4A, which allows the capacitive voltageV to again build up, the arc continues. The energy required to sustainthe arc has been reduced by thermionic emission and expansion of theelectrodes, and line current flows through the arc until the alternationof the line current goes through Zero at point 66. The are isextinguished as the current alternation goes through zero and the samesequence repeats itself during the following half cycles until thecapacitive voltage V drops b The value of magnet members 42 and 44, andtheir cooperation with electrodes 32 and 34 in protective system is,therefore, apparent. The magnet members 42 and 44 extinguish the areafter capacitance 12 has substantially discharged by moving the arcacross the face of electrodes 32 and 34 preventing thermionic emissionand prevent burning damage to the electrode surfaces. Without magnetmembers 42 and 44 the arc would not only be stationary, causing burningand melting of the electrodes, but the arc would be allowed to continue,due to the reduced amount of power required to sustain the are, from thepoint of the maximum capacitive voltage V max. until the currentalternation goes through zero. Operation without magnet members 42 and44 has no damping effect on the unstable circuit conditions offerro-resonance and sub-synchronous motor operation. Also, after a fewvoltage cycles, the electrodes would no longer be useful as a part ofthe protective system. The pits and burrs caused by the burning arcswould immediately change the effective gap and, therefore, completelychange the breakdown voltage of gap 40.

It is to be understood that although the voltage and current wave formsshown in FIGS. 4A, 4B, and 4C are shown in phase, in actual practice thevoltage and current may be out of phase.

The protective system 10 shown in FIG. 1 protects against excessivecapacitive voltage caused by overloads,

load changes, short circuits'andhas been proven to be completelyadequate in damping the conditions which produce and sustainsub-synchronous motor operation and ferro-resonance. FIGURE 5 showsoscillograms of capacitor voltage V and capacitor discharge current Iillustrating the damping of ferro-resonace. When the capacitor voltage Vreaches the breakdown voltage of the protective system 10, such as atpoints 51 and 53, the capacitor discharges, as shown at points 55 and57. Without the protective system 10, the capacitor voltageV would reachdamaging magnitudes, and the initial voltage wave form V would besustained as a steady state condition. However, with the protectivesystem 10, the maximum capacitor voltage V is controlled, and theunstable condition is quickly dampened, as shown by the reducedcapacitor voltage wave forms 59 and 61, and the substantial eliminationof disturbance at 63.

The theory behind the operation of protective system 10, and why magnetmembers 42 and 44 cooperate as they do with electrodes 42 and 44 toextinguish the arc after the discharge of capacitance 12, stopping anyflow of 60 cycle primary current through the electrodes, is due todeionization or magnetic blow-out. The rapid movement of the are overcool electrodes maintains the power required to sustain the arc at amaximum, thus quickly extinguishing the arc when the circuit power fallsbelow the sustaining value. The exact theory however, behind theexcellent results obtained by the protective system described herein indamping unstable circuit conditions is not known. However, in additionto the arc movementby magnet members 42 and 44, the discharge time ofthe capacitance 12 has been found to be one of the controlling factors.If the capacitive discharge time is very short, arc extinguishmentclosely follows the discharge of the ca- 8 pacitance. If the capacitivedischarge time is lengthened, the arc is not extinguished until muchlater in the half cycle, and the circuit does not dampen ferro-resonanceand sub-synchronous motor operation.

In examining the effect of capacitive discharge time, reference will bemade to FIGS. 2 and 2A. FIG. 2 is a schematic diagram which may be theequivalent circuit of FIG. 1, whereby the distributed capacitancebetween windings 16 and 18 is lumped into one capacitance 70 connectedin series circuit relation with primary Winding 18, or capacitance 70may represent an actual capacitor connected in series circuit relationwith primary winding 18 of transformer 14, as illustrated in FIG. 2A.The discharge time of capacitance 70 is determined by the dischargecircuit resistance 72, the value of the capacitance.

in the discharge circuit, and the value of the inductance in thedischarge circuit. The inductance can certainly be neglected wherecapacitance 70 represents an actual capacitor, and is insignificant whencapacitance 70 is produced by transformer winding arrangements becauseof the magnetic symmetry in the transformer windings. The resistance 72of the discharge circuit is variable and may be changed to observe theeffect of different dis-charge times of capacitance 70. Since resistance72 represents the total circuit resistance, its minimum value is theinternal resistance of the capacitance. For example, in a 7200 voltsystem, a capacitive reactance of 5%, an air gap of .015, and usingmagnetic members 42 and 44 which establish a 1000 gauss magnetic fieldperpendicular to the are, it was determined that the resistance of thedischarge path should be kept below 2 ohms, as starting at substantially2 ohms the system will not reliably recover from the ferro-resonancethat occurs at no load inrush. No load inrush is the most severe testthat the protective system 10 must handle, demonstrated by the fact thatwith a discharge circuit resistance of 2 ohms the protective system willstill handle load changes satisfactorily;

When the discharge circuit resistance was increased to approximately 4ohms, with a 1000 gauss magnetic field, a change of load very often putsthe system into ferroresonance and recovery was not reliable. However,to illustrate the complex interaction between circuit dischargeresistance, capacitive discharge time, and strength of the magneticfield, it was found that recovery from no-load inrush ferro-resonancecould be made'reliable in the above example by increasing the magneticfield to 2900 gauss. Therefore, in this example, if the resistance ofthe discharge circuit, including the resistance of the interleavedsections of the transformer windings, if the capacitance is created bywinding configurations and arrangements, or the internal resistance ofthe capacitor if an actual capacitor is used, is below two ohms, a 1000gauss magnetic field may be used and the protective system will operatesatisfactorily under all conditions, including no-load inrush, loadchanges and short time short circuit currents.

It is to be understood that the specific air gap, magnetic field,critical resistance values etc. given herein are for specific examples.Therefore, if capacitors having a different capacitive reactance areused, or the system voltage is different, the air gap required maychange. If the air gap changes, the critical value of discharge circuitresistance may change, as well as the desired magnitude of the magneticfield.

If the discharge circuit resistance cannot be reduced below the criticalvalue, the discharge current pulse width is increased and the arcbetween electrodes 32 and 34 of protective system 10 is not extinguishedat the end of the capacitive discharge, allowing 60 cycle per secondcurrent or line current to be conducted through the gap 40. Increasingthe magnetic field strength aids in extinguishing the arc, ashereinbefore shown. However, instead of resorting to very high magneticfields with a more violent arc movement and blowout action to extinguishthe are when the discharge circuit resistance exceeds the criticalvalue, another method has been found to be more suitable, which isillustrated schematically in FIG. 3.

'zero soon enough after gap breakdown to permit the arc to beextinguished before 60 cycle or line currentbegins to conduct throughthe gap 40. The addition of inductance 74 will effectively matcha-discharge circuit having relatively high internal resistance(approximately to 12 ohms) to the protector spark gap 40, allowing thearc to, t be extinguished before 60 cycle per second line current beginsto flow through the arc.

FIG. 6 illustrates an embodiment of the invention whereby a seriescapacitance 200 is utilized in a power distribution system, includingpower source 201 and load circuits 203, to aid voltage regulation, butis not directly associated with any one particular distributiontransformer. More specifically, protective system 10 is connected toprotect series capacitor 200, which is connected in power distributionsystem 202 to provide voltage regulation for a plurality of distributiontransformers 204. Transformer 206 is shown merely to illustrate thatthere may be other voltage transformations before the voltage is appliedto the distribution transformers 204. The operation of protective system10 is as hereinbefore described. The fact that current for a pluralityof distribution transformers, instead of for one particular transformer,flows through capacitor 200 does not affect the operation of theprotective system.

FIGS. 7, 8 and 9 show side and front elevations and the top view,respectively, of a practical embodiment 78 of the invention. FIG. 7 is aside elevation, partially in section, showing electrodes 80 and 82disposed to have a gap 84 of a predetermined length between adjacentelectrode surfaces. Magnet members 86 and 88 are disposed relative tothe gap 84 such that the magnetic field produced by magnet members 86and 84 will be substantially perpendicular to an are between electrodes80 and 82. The electrodes 80- and 82 and magnet members 86 and 88 aredisposed in a suitable casing 90, which may be constructed of aconductor of electricity such as aluminum or brass, or a non-conductormay be used if a separate ground lead is used, with said casing having acover or top 92. Enclosure 90 may be disposed on the outside of thecasing 93 of the inductive apparatus the protective apparatus is toprotect. One electrical connection from the electrical inductiveapparatus to one of the electrodes of the protective system 78 may bemade through insulating bushing member 94 which extends through thecasing 93 to the inductive apparatus. The conducting member 96 extendsthrough the bushing member 94, with connection being made to a conductorfrom the inductive apparatus at one end of conducting member 96 byfastening means 98. The other end of conducting member 96 may beconnected to electrode 80 through another conducting member 100 whichmay be secured to conducting member 98 by fastening means 102 and toelectrode 80 by suitable holding and locating means, such as springmember 104. The remaining electrical connection may be made by properlygrounding casing 90, which is directly connected to electrode 82.

FIGS. 8 and 9 show suitable hold-ing and locating means 110 and 112which may be used to hold electrodes 80 and 82 and magnetic members 86and 88 in the proper assembled relationship. FIG. 8 also shows how theelectrodes 80 and 82 may be tapered or rounded at points 114 and 114' toprevent arc concentration on sharp edges, and further cut back at points116, 116, and 118, 118'. This design of electrode is merely forillustrative purposes, however, with many different configurations beingequally suitable. In fact, excellent results have been obtained withrectangular shaped electrodes which were not tapered, rounded or cutback in any way.

When the over FIG. 10 illustrates the protective system 78 shown inFIGS. 7, 8 and 9 disposed relative to a transformer 120 which is shownpartially schematic. Transformer 120 may be of the conventional typehaving a high voltage winding 122 and a low voltage winding 124inductively disposed on a magnetic core 126, and disposed in a suitablemetallic casing or tank 128 containing the usual insulating dielectricand low voltage bushings 130 and 132 and high voltage 134, along with acapacitor 136 connected in series circuit relation with high'voltagewinding 122; or transformer 120 may be of the type whereby the seriescapacitance 136 is formed by using certain types and arrangements ofconductors and insulations of various windings. In any event, the highvoltage conductor 138 from high voltage bushing 134 is connected toterminal 140 of high voltage winding 132, and terminal 142 of highvoltage winding 122 is connected to protective device 78 throughconductor 144, as well as to ground 146 through capacitance 136. Theprotective device 78 is also connected to ground 146' such that when thevoltage across capacitor 136 exceeds the breakdown voltage of protectivedevice 78 the capacitance 176 will be effectively shunted, connectingterminal 142 of high voltage winding 122 directly to ground 146' for atime sufficient to allow capacitance 136 to discharge, as hereinbeforedescribed.

While protective device 78 in FIG. 10 is shown mounted external to thecasing 128 of transformer 120, it is to be understood that theprotective device 78 may be appropriately sealed and mounted inside thecasing.

Also, instead of air inside the protective casing of protective device78, it may be evacuated, or a gas such as sulfur hexafluoride (SPhydrogen, or one of the inert gases, may be used.

FIGS. 11 and 12 illustrate further embodiments the protective device maytake, using ring type magnets. FIG. 11 illustrates an arrangement shownin section, whereby circular electrode members 220 and 222 are disposedwith a predetermined gap 224 between adjacent surfaces, and ring typemagnet members 226 and 228 are disposed above and below the electrodevmembers 220 and 222, with the polarities of the magnet members being asillustrated with the north poles being on the outside periphery, or thesouth. poles may be on the outside periphery, to produce a magneticfieldv which will be substantially perpendicular to an arc between theelectrode members 228 and 222'. An arc between electrode members 220 and222, under influence of the magnetic field produced by magnet members220 and 222 will move in a circle, around said circular electrodemembers.

FIG. 12 illustrates an arrangement whereby circular electrode members230 and 232 are disposed with a predetermined gap or spacing 234 betweenadjacent surfaces, with ring type magnet members 236 and 248 disposedrelative to the internal and external diameters of electrode members 230and 232, respectively. The polarities of said magnet members should besuch that a magnetic field is created across the gap 234 which will besubstantially perpendicular to an are formed between electrode members230 and 232. Like the embodiment shown in FIG. 11, an arc betweenelectrode members 230 and 232 will be moved in a circle around saidelectrode members.

The protective system described herein has many advantages, in additionto its very low cost which makes its use with relatively inexpensivedistribution transformers practical. One of the advantages is the factthat protective device effectively shunts the series capacitance andextinguishes the arc before 60 cycle or a line current can flow throughthe arc, thus immediately damping unstable circuit conditions. Anotheradvantage is the fact that the protective device operates during thefirst half cycle of over voltage and can repeatedly and automaticallyperform its protective function without maintenance. voltage on thecapacitance ceases, the

protective device no longer shunts the capacitance and is ready to awaitthe next over voltage condition. Another advantage is the small size,lack of moving parts, and rugged construction of the protective system,which contribute to a substantially maintenance free system and allowsit to be conveniently mounted relative to the apparatus it is toprotect.

Since numerous changes may be made in the abovedescribed apparatus anddifferent embodiments of the invention may be made without departingfrom the spirit thereof, it'is intended that all matter contained in theforegoing description or shown in the accompanying drawings shall beinterpreted as illustrative, and not in a limiting sense.

We claim as our invention:

1. Electrical inductive apparatus comprising windings arranged toprovide a predetermined capacitance, protective means connected acrosssaid capacitance, said protective means comprising electrode meanshaving predetermined minimum dimensions and disposed to form a gap whichwill prevent electrical conduction below a predetermined voltage andallow conduction in the form of an arc above the predetermined voltage,and means disposed to move the arc in the gap provided by said electrodemeans along one of the dimensions of said electrode means, thepredetermined minimum dimensions of said electrode means being selectedto cause the arc to extinguish when said capacitance has substantiallydischarged, the predetermined minimum dimensions of said electrode meansbeing determined by the discharge time of said capacitance and the speedof the arc.

2. Electrical inductive apparatus comprising windings interleaved toprovide a predetermined capacitance, protective means connected acrosssaid capacitance, said protective means comprising electrode meanshaving predetermined minimum dimensions and disposed to form a gap whichprevents electrical conduction'below a predetermined voltage and allowsconduction in the form of an arc above the predetermined voltage, andmeans disposed to provide a magnetic field in said gap which moves theare along one of the dimensions of said electrode'means, thepredetermined minimum dimensions of said electrode means being selectedto cause the arc to.

extinguish when said capacitance has substantially discharged, thepredetermined minimum dimensions of said electrode means beingdetermined by the discharge time of said capacitance and the strength ofthe magnetic field.

3. Electrical inductive apparatus comprising primary and secondarywindings, capacitance means connected in series circuit relation withsaid primary winding, electrode means having predetermined minimumdimensions and connected across said capacitance means, said electrodemeans being disposed to provide a predetermined gap which preventscurrent fiow below a predetermined voltage and allows current flow inthe form of an arc above the predetermined voltage, and means disposedto move the arc in the gap provided by said electrode means along one ofthe dimensions of said electrode means, the predetermined minimumdimensions of said electrode means being selected to cause the arc toextinguish when said capacitance means has substantially discharged, thepredetermined minimum dimensions of said electrode means beingdetermined by the discharge time of said capacitance means and the speedof the arc.

4. A transformer comprising primary and secondary windings forconnection to a source of alternating potential and a load circuit, saidprimary winding being arranged to provide a predetermined capacitance inseries circuit relation with said primary winding, electrode meanshaving predetermined minimum dimensions and connected across saidcapacitance, said electrode means being disposed to provide apredetermined air gap, means disposed to provide a magnetic field in theair gap provided by said electrode means, the air gap provided by saidelectrode means breaking down and conducting electricity in the form ofan are when the voltage across said capacitance reaches a predeterminedmagnitude, said arc being moved along the gap in the direction of one ofthe dimensions of said electrode means by the action of the magneticfield, the predetermined minimum dimensions of said electrode meansbeing selected to cause the arc to extinguish when said capacitance hassubstantially discharged, the predetermined minimum dimensions of saidelectrode means being determined by the discharge time of saidcapacitance and the strength of the magnetic field.

5. A transformer comprising primary and secondary windings forconnection to a source of alternating potential and a load circuit,capacitance means connected in series circuit relation with said primarywinding, electrode means having predetermined minimum dimensionsconnected across said capacitance means and disposed to provide apredetermined gap, the gap provided by said electrode means breakingdown and conducting electricity in the form of an arc when the voltageacross said capacitance means exceeds a predetermined magnitude, meansdisposed to provide a magnetic field in the gap provided by saidelectrode means substantially perpendicular to the arc, the are beingmoved along the gap in the direction of one of the dimensions of saidelectrode means by the action of said magnetic field, the predeterminedminimum dimensions of said electrode means being selected to cause thearc to extinguish when said capacitance means has substantiallydischarged, the predetermined minimum dimensions of said electrode meansbeing determined by the discharge time of said capacitance means and thestrength of the magnetic field.

6. A transformer comprising primary and secondary windings forconnection to a source of alternating potential and a load circuit,capacitance means connected in series circuit relation with said primarywinding, inductance means, electrode means having predetermined minimumdimensions and disposed to provide a predetermined gap, said inductancemeans and said electrode means being connected serially across saidcapacitance means, the gap provided by said electrode means allowingsaid capacitance means to discharge in the form of an arc when thevoltage across said capacitance means reaches a predetermined magnitude,means disposed to provide a magnetic field in said gap having a fieldstrength sufficient to move the arc in the gap provided by saidelectrode means along one of the dimensions of said electrode means,said inductance means causing the discharge current of said capacitancemeans to be oscillatory and having a predetermined natural frequency,the predetermined minimum dimensions of said electrode means beingdetermined by the discharge time of said capacitance means and thestrength of the magnetic field, the magnetic field and said electrodemeans extenguishing the are at a current zero of the oscillatorydischarge current when said capacitance means has substantiallydischarged.

7. A transformer comprising a plurality of electrical windings, certainof said windings being interleaved to provide a predeterminedcapacitance, inductance means, electrode means having predeterminedminimum dimensions and disposed to provide a predetermined gap, saidinductance means and said electrode means being connected seriallyacross said capacitance, the gap provided by said electrode meansallowing said capacitance to discharge in the form of an are when thevoltage across said capacitance reaches a predetermined magnitude, meansdisposed to provide a magnetic field in said gap having a field strengthsufficient to move the arc in the gap provided by said electrode meansalong one of the dimensions of said electrode means, said inductancemeans causing the discharge current of said capacitance to beoscillatory and having a predetermined natural frequency, thepredetermined minimum dimensions of said electrode means beingdetermined by the discharge time of said capacitance and the strength ofthe magnetic field, the magnetic field and said electrode meansextinguishing the arc at a current zero of the oscillatory dischargecurrent when said capacitance has substantially discharged.

8. A power distribution system comprising a plurality of distributiontransformers having primary and secondary windings, capacitance means,said capacitance means being connected in circuit relation with saidplurality of distribution transformers, with the electrical energyflowing through said capacitance means also flowing through the primarywindings of said distribution transformers, electrode means havingpredetermined minimum dimensions and disposed to provide a predeterminedgap, said electrode means being connected across said capacitance means,the gap provided by said electrode means allowing said capacitance meansto discharge in the form of an are when the voltage across saidcapacitance means reaches a predetermined magnitude, means disposed tomove the are along one of the dimensions of said electrode means, thepredetermined minimum dimensions of said electrode means beingdetermined by the discharge time of said capacitance means and the speedof the arc, said electrode means and the movement of the arc cooperatingto extinguish the are when said capacitance means has substantiallydischarged.

9. Protective means for damping unstable circuit conditions andpreventing over-voltages when utilizing series connected capacitance ina power distribution system, comprising electrode means havingpredetermined minimum dimensions and disposed to provide a predeterminedgap, said electrode means being adapted for connection across thecapacitance, said electrode means discharging the capacitance in theform of an arc across the gap when the voltage across the capacitancereaches a predetermined magnitude, means disposed to move the arc in thegap provided by said electrode means along one of the dimensions of saidelectrode means, the predetermined minimum dimensions of said electrodemeans being determined by the discharge time of the capacitance and themovement speed of the arc, the movement speed of the arc and thepredetermined minimum dimensions of said electrode means cooperating tocause the arc to extinguish when the capacitance has substantiallydischarged.

10. Protective means for damping unstable circuit conditions andpreventing overvoltages when utilizing series connected capacitance in apower distribution system, comprising substantially circular shapedelectrode means having predetermined minimum dimensions and disposed toprovide a predetermined gap, said electrode means being adapted forconnection across the capacitance, said means to extinguish when thecapacitance has substantially discharged.

11. Protective means for capacitance connected serially with inductiveapparatus, comprising electrode members having predetermined minimumdimensions and disposed to provide a predetermined gap, said electrodemembers being adapted for connection across the capacitance, saidelectrodemembers discharging the capacitance in the gap in the form ofan arc when the voltage across the capacitance reaches a predeterminedmagnitude, magnet members, said magnet members being disposed to providea magnetic field in said gap which moves the arc in the gap along one ofthe dimensions of said electrode members, the predetermined minimumdimensions of said electrode members being determined by the dischargetime of the capacitance and the strength of the magnetic field, themagnetic field and said electrode members causing the arc in the gapprovided by said electrode members to extinguish when the capacitancehas substantially discharged.

References Cited by the Examiner UNITED STATES PATENTS 787,990 4/1905Murphy 313 156 2,575,060 9/1951 Mathias. 2,664,525 12 1953 DlebOld 317-12 2,677,032 4/1954 Wells. 2,725,446 9/1955 Slepian. 2,900,578 8/1959Marbury 317-12 FOREIGN PATENTS 1,072,308 3/1954 France.

SAMUEL BERNSTEIN, Primary Examiner. R. V. LUPO, Assistant Examiner.

1. ELECTRICAL INDUCTIVE APPARATUS COMPRISING WINDINGS ARRANGED TOPROVIDE A PREDETERMINED CAPACITANCE, PROTECTIVE MEANS CONNECTED ACROSSSAID CAPACITANCE, SAID PROTECTIVE MEANS COMPRISING ELECTRODE MEANSHAVING PREDETERMINED MINIMUM DIMENSIONS AND DISPOSED TO FORM A GAP WHICHWILL PREVENT ELECTRICAL CONDUCTION BELOW A PREDETERMINED VOLTAGE ANDALLOW CONDUCTION IN THE FORM OF AN ARC ABOVE THE PREDETERMINED VOLTAGE,AND MEANS DISPOSED TO MOVE THE ARC IN THE GAP PROVIDED BY SAID ELECTRODEMEANS ALONG ONE OF THE DIMENSIONS OF SAID ELECTRODE MEANS, THEPREDETERMINED MINIMUM DIMENSIONS OF SAID ELECTRODE MEANS BEING SELECTEDTO CAUSE THE ARC TO